1. Field of the Invention
The present invention relates to a pickup lens with a phase compensator that can be used for a multiwavelength optical system using a plurality of kinds of monochromatic light such as a compatible recording and reproduction apparatus that is compatible with different types of optical recording media such as compact discs (CDs) including CD-R, digital versatile discs (DVDs), Blu-ray and High-Definition DVD (HD-DVD) and an optical pickup using the same.
2. Description of Related Art
Compatible optical disc apparatus capable of recording or reproducing data on different types of optical discs such as CD and DVD with one system have been proposed.
In order to record or reproduce information signals stored on optical discs such as CD and DVD (which is hereinafter collectively called the optical disc), the compatible optical disc apparatus needs to focus a laser beam from a light source on an information recording surface of each optical disc through a transparent substrate. However, the wavelength differs between a laser beam used for recording or reproducing CD and a laser beam used for recording or reproducing DVD. Further, while CD has a transparent substrate of 1.2 mm in thickness, DVD has that of 0.6 mm. Because of aberrations caused by these reasons, use of a conventional condenser lens for both CD and DVD in the compatible optical disc apparatus fails to focus each laser beam used for CD and DVD on the information recording surface of each optical disc close to the diffraction limit.
A recently proposed apparatus compatible with optical discs capable of ultra high density recording, such as Blu-ray and HD-DVD, uses a blue laser with a wavelength of approximately 405 nm for recording and reproduction of information. Therefore, future compatible optical disc apparatus are expected to record or reproduce data on not only CD and DVD but also optical discs for ultra high density recording. Thus, though conventional compatible optical disc apparatus allows for two different light source wavelengths and two different thicknesses of transparent substrates, future compatible optical disc apparatus needs to allow for at most three different light source wavelengths and at most three different thicknesses of transparent substrates.
To meet this need, a compatible optical disc apparatus may have a plurality of condenser lenses to prevent aberrations for different types of optical discs in a pickup so that the condenser lenses are changed in accordance with the type of the optical disc in use. Alternatively, it may have a plurality of pickups for different types of optical discs so that the pickups are changed in accordance with the type of the optical disc in use. However, in terms of cost and size reduction, it is preferred to use one lens as a condenser lens for any type of optical disc.
An example of such a condenser lens is described in Japanese Unexamined Patent Publication No. 2004-6005 (Ota et al.) According to an aspect of Ota et al., a device includes a light source with a wavelength λ1 for recording and reproducing information on a second optical medium, a light source with a wavelength λ2 (λ1<λ2) for recording and reproducing information on a first optical medium, a light source with a wavelength λ3 (λ2<λ3) for recording and reproducing information on a third optical medium having a thicker substrate than the first and the second optical medium, and a condenser lens for focusing a light beam from each light source on each optical medium. This device applies a light beam with the wavelength λ1 as parallel light to the condenser lens when recording or reproducing data on the second optical medium. It applies a light beam with the wavelength λ2 as parallel light to the condenser lens when recording or reproducing data on the first optical medium. Further, it applies a light beam with the wavelength λ3 as divergent light to the condenser lens when recording or reproducing data on the third optical medium.
The condenser lens taught by Ota et al. has a diffractive lens structure where minute annular zone steps are thickly formed on one side surface of a refractive lens having a positive refractive power. The diffractive lens structure is designed so as to focus a laser beam with the wavelength λ1 that is incident on the condenser lens as a parallel light beam, which is referred to herein as the infinite system, to an information recording surface of the second optical medium having a substrate of a small thickness and to focus an infinite laser beam with the wavelength λ2 to an information recording surface of the first optical medium having a substrate of the same thickness.
On the other hand, since the condenser lens does not allow for the wavelength λ3, wavefront aberration occurs when recording or reproducing information on the third optical medium by using a laser beam with the wavelength λ3. Thus, the laser beams is not collimated to a parallel beam but is incident on the condenser lens as divergent light, which is referred to herein as the finite system. This technique uses the fact that spherical aberration changes by changing a degree of divergence of the incident light, which is an object distance for the condenser lens in geometric optical terms.
Another example of the condenser lens is described in Japanese Unexamined Patent Publication No. 2004-79146 (Kimura et al.). According to an aspect of Kimura et al., an apparatus includes a light source with a wavelength λ1 for recording and reproducing information on a first optical medium, a light source with a wavelength λ2 (λ1<λ2) for recording and reproducing information on a second optical medium, a light source with a wavelength λ3 (λ2<λ3) for recording and reproducing information on a third optical medium, and a condenser lens for focusing a light beam from each light source on each optical medium. This apparatus applies a light beam with the wavelength λ1 as parallel light to the condenser lens when recording or reproducing data on the first optical medium. It applies a light beam with the wavelength λ2 as divergent light to the condenser lens when recording or reproducing data on the second optical medium. Further, it applies a light beam with the wavelength λ3 as divergent light to the condenser lens when recording or reproducing data on the third optical medium.
This condenser lens has a diffractive lens structure where minute annular zone steps are thickly formed on one side surface of a refractive lens having a positive refractive power just like the above case. This condenser lens, however, is designed so that the wavelength λ1 is infinite and the wavelength λ2 and λ3 are finite. If the wavelength λ2 and λ3 are finite so as to apply divergent light beams to the second and the third optical disc, it is possible to reduce the aberration that occurs due to a difference in substrate thickness of different kinds of optical discs, which the diffractive structure needs to reduce. This allows increasing the interval between adjacent loop zones and thereby reducing a decrease in diffraction efficiency due to errors in manufacturing the loop zone shape.
According to another aspect of Kimura et al., an apparatus includes a light source with a wavelength λ1 for recording and reproducing information on a first optical medium, a light source with a wavelength λ2 (λ1<λ2) for recording and reproducing information on a second optical medium, a light source with a wavelength λ3 (λ2<λ3) for recording and reproducing information on a third optical medium, and a condenser lens for focusing a light beam from each light source on each optical medium. This apparatus applies a light beam with the wavelength λ1 as parallel light to the condenser lens when recording or reproducing data on the first optical medium. It applies a light beam with the wavelength λ2 as parallel light to the condenser lens when recording or reproducing data on the second optical medium. Further, it applies a light beam with the wavelength λ3 as parallel light to the condenser lens when recording or reproducing data on the third optical medium.
This condenser lens is designed so that wavefront aberration is small only for the wavelength λ1 and the substrate thickness of the first optical medium, and it does not have a diffractive lens structure where minute annular zone steps are thickly formed on one side surface of a refractive lens having a positive refractive power. Though the wavefront aberration therefore occurs when recording information on the second optical medium with the wavelength λ2, this technique corrects the wavefront aberration by using a coupling lens having a diffractive structure composed of a plurality of concentric loop zones in a light path through which only the light with the wavelength λ2 passes. Similarly, though the wavefront aberration occurs when recording information on the third optical medium with the wavelength λ3, this technique corrects the wavefront aberration by using a coupling lens having a diffractive structure composed of a plurality of concentric loop zones in a light path through which only the light with the wavelength λ3 passes. As yet another aspect, Kimura et al. describes a technique that uses a dual wavelength laser of wavelengths λ2 and λ3 to share a coupling lens having a diffractive structure composed of a plurality of concentric loop zones in common for the wavelengths λ2 and λ3.
Since the above techniques use a condenser lens in common, it is possible to eliminate the need for means to replace members used for each type of optical disc including condenser lenses, which reduces costs and simplifies the structure.
However, the present invention has recognized that the above techniques have the following problems. Specifically, though the technique taught by Ota et al. corrects aberrations by using the finite system when recording or reproducing information on the third optical medium with the wavelength λ3, it is difficult to share components with the infinite optical system used when recording and reproducing information on the second optical medium with a laser beam of the wavelength λ1 and when recording and reproducing information on the first optical medium with a laser beam of the wavelength λ2. Further, in a case of using a three-wavelength laser having wavelengths of λ1, λ2 and λ3 as one element, it is difficult to make finite system only when recording and reproducing information on the third optical medium with a laser beam of the wavelength λ3, which hinders achievement of a simple optical system. This is the same for the case of making finite system only when recording and reproducing information on the first optical medium with a laser beam of the wavelength λ1 as taught by Kimura et al.
Furthermore, in a case of performing tracking servo by applying divergent light to a condenser lens and mounting the condenser lens onto an actuator, aberrations specific to the finite system that are caused by misalignment of the optical axis of the condenser lens and the optical axis of incident light occur, thereby failing to sufficiently focus laser beams on the information surface of an optical disc.
Kimura et al. also teach the technique to make all the structure with finite system as described above. However, it requires inserting a coupling lens for correcting wavefront aberrations in a light path through which only the light with the wavelength λ2 or λ3 passes. Further, use of the three-wavelength laser complicates the structure of the optical system.